Vortex generators on rotor blades to delay an onset of large oscillatory pitching moments and increase maximum lift

ABSTRACT

An airborne mobile platform generally includes a plurality of rotating rotor blades operating in an airflow that forms a boundary layer on each of the rotor blades. At least one of the rotor blades includes a section that encounters the airflow that includes an unsteady subsonic airflow having at least a varying angle of attack. At least one of the rotor blades also includes one or more vortex generators on the at least one of the rotor blades that generate a vortex that interacts with the boundary layer to at least delay an onset of separation of the boundary layer, to increase a value of an unsteady maximum lift coefficient and to reduce a value of an unsteady pitching moment coefficient for the section.

FIELD

The present teachings relate to an airborne mobile platform havingrotating rotor blades and more particularly relate to vortex generatorson each rotor blade of a rotorcraft to reduce the onset of boundarylayer separation and dynamic pitching moments in an unsteady subsonicairflow.

BACKGROUND

There are many airborne mobile platforms that can employ one or moreairfoils to supply, lift and/or thrust. In a fixed wing aircraft, forexample, the wings (i.e., the airfoils) can experience relatively steadyairflow. At relatively high angles of attack (i.e., orientation of theairfoil to the airflow) and/or relatively high airflow velocities, aboundary layer can sufficiently detach from a surface of the wingcausing a stall condition. In the stall condition, the wings canexperience a loss in lift.

Unlike wings on the fixed-wing aircraft, rotor blades of a rotorcraftcan rotate with a rotor hub to which the rotor blades are connected. Therotating rotor blades are subject to cyclical variations in blade pitchangle, as well as unsteady high-subsonic airflow that can includerelatively high frequency and relatively large amplitude variations inangle of attack and relatively rapid and periodic changes in an airflowvelocity at one or more sections of each of the rotor blades. Rotorblades rotating through the unsteady airflow can have an increase in themaximum achievable lift (i.e., increase in airfoil section C_(max)) dueto the unsteady variations in angle of attack.

While there can be an increase in the maximum achievable lift, when therotor blade does stall (i.e., lift stall), the rotor blade canexperience a relatively large nose-down pitching moment. The relativelylarge nose-down pitching moment (i.e., moment stall) which usuallyprecedes the lift stall can cause large vibratory loads in rotor bladecontrols and the rotor hub. Because of these vibratory loads, the speed,weight, altitude and/or other performance parameters of the rotorcraftmay need to be limited so that these high vibratory loads can beavoided. Moreover, flight time in such conditions can reduce the life ofthe rotor hub and the rotor blade controls and can increase maintenancecosts.

Typically, the solidity of the rotor blade can be increased to delay theonset of boundary layer separation, i.e., the stall condition.Increasing rotor solidity can include increasing a chord of the rotorblade or increasing the number of blades. For certain overall weightand/or operating speeds, the increase in the solidity of the rotor bladecan reduce a value of a local section lift coefficient (i.e., decreaseC_(l)) at certain local rotor sections below the maximum value ofachievable lift (i.e., C_(lmax)). By doing so, the onset of the stallcondition can be delayed. While the stall condition can be delayed, therotor blade can, nevertheless, stall. Moreover, increasing the solidityof the rotor blade can increase the magnitude of the pitching moment ofthe rotor blade by a square of the chord length (i.e., (pitchingmoment)˜(chord length)²).

To address the increased magnitude of the pitching moment, the rotorblade airfoils can be implemented with trailing edge tabs and/or arelatively moderate camber. The trailing edge tabs can be set at anegative angle, i.e., upward from the trailing-edge. Alternatively, therotor blade airfoils can be designed to have negative camber (i.e.,reverse camber) in a region of the trailing edge. The variouscombinations of changes to solidity and camber and the addition oftrailing edge tabs can delay the onset of stall and can reduce themagnitude of the pitching moments due to the stall condition.

The various combinations can, however, add to the complexity and weightof the rotor blades especially increasing the number of rotor blades.Increasing the solidity of the rotor blades and/or increasing the numberof the rotor blades can require more engine power to overcome increasedprofile drag produced by the rotor blades, as profile drag can beproportional to the blade area. Increased rotor blade solidity and/orcamber and/or solidity can increase the weight of the rotor blades, therotor hub, the rotor blade controls and associated structures of therotorcraft. While the above rotor blade configurations remain useful fortheir intended purposes, there remains room in the art for improvement.

SUMMARY

The various aspects of the present teachings generally include anairborne mobile platform that generally includes a plurality of rotatingrotor blades operating in an airflow that forms a boundary layer on eachof the rotor blades. At least one of the rotor blades includes a sectionthat encounters the airflow that includes an unsteady subsonic airflowhaving at least a varying angle of attack. At least one of the rotorblades also includes one or more vortex generators on the at least oneof the rotor blades that generate a vortex that interacts with theboundary layer to at least delay an onset of separation of the boundarylayer, to increase a value of an unsteady maximum lift coefficient andto reduce a value of an unsteady pitching moment coefficient for thesection.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the present teachings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the present teachings in any way.

FIG. 1 is a diagram of a top view of a portion of a rotorcraft or othersuitable airborne mobile airborne platform having rotor blades that canextend from a rotor hub in accordance with the present teachings.

FIG. 2A is a diagram of a section of a rotor blade having a mechanicaltype of vortex generator connected thereto in accordance with one aspectof the present teachings.

FIG. 2B is similar to FIG. 2A and shows a section of a rotor blade withone type of a fluidic vortex generator in accordance with another aspectof the present teachings.

FIG. 2C is similar to FIG. 2B and shows another type of fluidic vortexgenerator in accordance with a further aspect of the present teachings.

FIG. 2D is a diagram of a section of a rotor blade having various typesof vortex generators connected thereto in accordance with yet anotheraspect of the present teachings.

FIG. 3A is a diagram of a side view of a rotor blade with one or morevortex generators attached to a top surface of the rotor blade to delayon onset of separation of the boundary layer in accordance with thepresent teachings.

FIG. 3B is a diagram of a side view of a rotor blade with one or morevortex generators attached to a bottom surface of the rotor blade todelay on onset of separation of the boundary layer in accordance withthe present teachings.

FIG. 4 is a diagram of a section of a rotor blade having a vortexgenerator configured as a vane that can be moved between an extendedcondition and a retracted condition and/or can be selectively yawed inaccordance with the present teachings.

FIG. 5 is a diagram of an exemplary control system for one or morevortex generators in accordance with the present teachings.

FIG. 6 is a diagram showing values of lift coefficient and angle ofattack for an exemplary rotor blade having vortex generators inaccordance with the present teachings.

FIG. 7 is a diagram showing values of pitching moment coefficient andangle of attack for an exemplary rotor blade having vortex generators inaccordance with the present teachings.

FIG. 8 is a diagram showing a rotor blade airfoil showing a baselinebluntness and an altered bluntness that can be combined with one or morevortex generators so that the velocity profile over airfoil design toincrease the benefits provided by the vortex generators in accordancewith the present teachings.

FIG. 9 is a diagram of a section of a rotor blade having pairs of amechanical type of vortex generator connected thereto in accordance witha further aspect of the present teachings

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present teachings, their application or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

The various aspects of the present teachings can be applicable to any ofa wide range of airborne mobile platforms. The teachings can beparticularly useful with rotorcrafts such as helicopters, tilt rotors,autogiros, etc. The present teachings are also applicable to bothunmanned and manned aircraft that can be controlled directly, remotely,via automation, and/or one or more suitable combinations thereof. Thevarious aspects of the present teachings can be applicable to any of awide range of lift producing and/or thrust producing surfaces such asmain rotors, secondary main rotors, rear rotors, etc. Accordingly,specific references to an airfoil and/or to rotor blades herein shouldnot be construed as limiting the scope of the present teachings to thosespecific implementations.

Moreover, certain terminology can be used for the purpose of referenceonly and need not limit the present teachings. For example, terms suchas “upper,” “lower,” “above” and “below” can refer to directions in thedrawings to which reference is made. Terms such as “front,” “back,”“rear,” and “side” can describe the orientation of portions of thecomponent within a consistent but arbitrary frame of reference which canbe made more clear by reference to the text and the associated drawingsdescribing the component under discussion. Such terminology may includethe words specifically mentioned above, derivates thereof and words ofsimilar import. Similarly, the terms “first,” “second” and other suchnumerical terms referring to structures, systems and/or methods do notimply a sequence or order unless clearly indicated by the context.

In accordance with various aspects of the present teachings and withreference to FIG. 1, one or more airborne mobile platforms 10, such asan airplane, a helicopter, an autogiro, a tilt rotor, etc., can employrotor blades 12 to create lift and/or thrust. In one example, arotorcraft 14 can have the rotor blades 12 that can extend from a rotorhub 16. Each of the rotor blades 12 can have a chord and a span. Each ofthe rotor blades 12 can couple to the rotor hub 16 at a blade root 18that is distal from a blade tip 20 in a spanwise direction.

The rotorcraft 14 can travel in generally a forward direction. In thisregard, one of the rotor blades 12 can be in an advancing condition 22and another one of the rotor blades 12 can be in a retreating condition24. Each of the rotor blades 12 experiences an airflow 26 that can beaffected by an immediately preceding rotor blade, as the rotor blades 12can travel in their circular path, i.e., a rotor disc 28. In thisregard, each of the rotor blades 12 can experience unsteady airflowconditions arising from application of controlled periodic changes inblade pitch (i.e., cyclic pitch) and general airflow disturbances causedby the wakes of the other blades or other parts of the rotorcraft 14.These unsteady airflow conditions can cause sections of the rotor blade12 to experience relatively high frequency and/or for relatively largeamplitude variations in angle of attack and/or Mach number. The Machnumber of the airflow 26 can be subsonic. In one example, the operatingparameters of the rotorcraft 14 can include the Mach number of theairflow 26 being in a range from about 0.2 to 0.8 on the advancing blade(i.e., one of the rotor blades 12 in the advancing condition 22) and 0to 0.6 on the retreating blade, (i.e., one of the rotor blades 12 in theretreating condition 24).

One or more vortex generators 30 can be implemented on one or more ofthe rotor blades 12 in various forms and/or at various predeterminedpositions. In general and as shown in FIGS. 3A and 3B, vorticesgenerated by the vortex generators 30 increase the reluctance of aboundary layer 32 to separate from the rotor blade 12 under conditionsof high amplitude and/or high frequency changes in airfoilangle-of-attack. By keeping the boundary layer 32 substantially attachedto the rotor blade 12, as shown in FIGS. 3A and 3B, the rotor blade 12can tolerate greater variations of angle-of-attack and Mach numberbefore the rotor blade 12 enters dynamic moment stall. This canespecially be so for the rotor blade 12 in the retreating condition 24.In this regard, the relatively large and dynamic pitching moments can bereduced or avoided by reducing and/or avoiding the onset of stall.

More specifically, the unsteady airflow experienced by the rotor blades12 can establish high frequency variations in angle-of-attack and/orMach number over one or more sections of the rotor blades 12, which canresult in the rotor blade 12 experiencing varying values of the locallift coefficient (i.e., C_(l)) for the section of the rotor blade 12.The vortices from the vortex generators 30, however, can increase thelocal values of the maximum lift coefficient (i.e., C_(lmax)) and delaythe abrupt change in section pitching moment for that section of therotor blade 12. In this regard, the values of the lift coefficient canbe maintained below the values of the maximum lift coefficients. Inanother instance, the values of lift coefficient can be increased tosuch a point that the boundary layer 32 can separate from the rotorblade 12, in other words, encounter the stall condition. When the rotorblade 12 does experience the stall condition, the pitching moments dueto the stall condition can be reduced relative to pitching moments onrotor blades 12 whose solidity and/or camber had been modified to delaythe onset of boundary layer 32 separation and moreover have notimplemented the vortex generators 30 in accordance with the presentteachings.

As shown in FIG. 2D, the vortex generators 30 can be mechanical and/orfluidic devices that can be deployed on the rotor blades 12 in certainpredetermined configurations. As shown in FIGS. 2A and 4, mechanicalvortex generators 34 can be devices that physically extend into theairflow 26, such as a tab, a vane, etc. As shown in FIGS. 2B and 2C,fluidic vortex generators 36 can be devices that can inject a jet flowinto and/or extract the jet flow from the airflow 26 such aspiezoelectric pulse jets, zero net mass jets, etc. In FIG. 2B, thefluidic vortex generators 36 can be an oval or round fluidic vortexgenerator 36a (e.g., an orifice associated with one of the fluidicvortex generators 36 is oval or round). In FIG. 2C, the fluidic vortexgenerators 36 can be rectangular fluidic vortex generators 36b (e.g., anorifice associated with one of the fluidic vortex generators 36 isrectangular). The vortex generators 30 can all be a single type ofvortex generator 30 (e.g., all mechanical vortex generators 34 as shownin FIG. 2A). Alternatively, one or more types of vortex generators 30that can be employed on each or all of the rotor blades 12 and/or one ormore suitable combinations thereof, as shown in FIG. 2D.

The mechanical vortex generators 34 can be fixed (i.e., not movablerelative to the rotor blade 12) or can be adjustable. In one example andwith reference to FIG. 4, the vortex generators 30 can include one ormore vanes 38 that can be placed at specific chord and span positionsalong the rotor blade 12 a. The vanes 38 can be stationary, the vanes 38can move relative to the rotor blade 12 a and a combination thereof.Movement of the vanes 38 can include various deviations in pitch, rolland/or yaw relative to an initial position. In one example, the vanes 38can be fixed in the direction of pitch and roll but can be yawed (i.e.,generally rotation about a z-axis 40 that can be generally normal to aground blade section chord line 42). The yawing of each vane 38 can bebased on an angle of attack of the rotor blade 12 a, the airflowvelocity, the position of the rotor blade 12 a on which the vortexgenerators 30 can be attached in the rotor disc 28 (FIG. 1) (e.g., theblade being in the retreating condition 24 as opposed to the advancingcondition 22) and/or one or more combinations thereof.

The vanes 38, whether fixed and/or adjustable can be extended from, andretracted into, a surface 44 of the rotor blade 12 a. Moreover, thevanes 38 and/or one or more other suitable vortex generators 30 can beimplemented on a top surface 46 (FIG. 3A) and/or a bottom surface 48(FIG. 3B) of the rotor blade 12,12 a and/or combinations thereof. Theyawing, extension, retraction and/or one or more combinations thereof ofone or more of the vanes 38 can be based on an angle of attack of therotor blade 12, the velocity of the airflow 26, the position of therotor blade 12 on which one or more of the vortex generators 30 areattached in the rotor disc 28 (FIG. 1) and/or one or more combinationsthereof.

In an example implementing fluidic vortex generators 36, one or more ofthe fluidic vortex generators 36 can be placed at certain chord and spanpositions. In one example and with reference to FIG. 5, the vortexgenerators 30 can be arranged so that multiple vortex generators 30 onthe rotor blade 12 can be divided into a first set 100, a second set102, etc., which are part of a closely spaced array of fluidic vortexgenerators. Each of the sets 100,102 can be in an active condition(e.g., oscillating between injecting and extracting the jet flow) or inan inactive condition (e.g., neither injecting nor extracting). Wheneach of the sets 100, 102, etc. is in the active condition, each of thefluidic vortex generators 36 inject and/or extract the jet flow in asimilar or dissimilar fashion relative to other fluidic vortexgenerators 36 in the same set. For example and as applicable, the firstset 100 can all be in the active condition but certain fluidic vortexgenerators 36 in the first set 100 can inject and/or extract the jetflow differently than other fluidic vortex generators 36 in the firstset 100.

The first set 100 and the second set 102, etc. of the fluidic vortexgenerators 36 can be associated with certain chord positions and/or spanpositions so that activating and deactivating certain fluidic vortexgenerators 36 can correspond to certain locations on the rotor blade 12.In addition, as flight conditions and/or rotor blade 12 orientationchange (i.e., change in an angle of incidence), the amount of eitheractive or inactive fluidic vortex generators 36 can change. Further, thefashion in which each of the fluidic vortex generators 36 can injectand/or extract the jet flow (e.g., change in magnitude, frequency, pulsewidth, etc.) can change as flight conditions and/or rotor blade 12orientation change.

In one example, the fluidic vortex generators 36 can include one or moreoscillating jets that can be similar to those disclosed in the followingcommonly assigned United States Patents: U.S. Pat. No. 6,899,302, titledMethod and Device for Altering the Separation Characteristics of Flowover an Aerodynamic Surface via Hybrid Intermittent Blowing and Suction,issued May 31, 2005; U.S. Pat. No. 6,866,234, titled Method and Devicefor Altering the Separation Characteristics of Air-flow over anAerodynamic Surface via Intermittent Suction, issued Mar. 15, 2005; U.S.Pat. No. 6,713,901, titled Linear Electromagnetic Zero Net Mass JetActuator, issued Mar. 30, 2004; and U.S. Pat. No. 6,471,477, titled JetActuators for Aerodynamic Surfaces, issued Oct. 29, 2002. The abovereferences are hereby incorporated by reference as if fully set forthherein.

The mechanical vortex generators 34 and/or the fluidic vortex generators36 can be controlled by a controller 104 that can be integral to or inaddition to existing avionic systems 106 or other suitable navigational,flight control, flight communication, etc. systems in the rotorcraft 14(FIG. 1). As such, the pilot (whether human and/or computer) candirectly and/or indirectly control the switching of each of the fluidicvortex generators 36 between the active and inactive conditions and/orcan control the fashion in which each of the fluidic vortex generators36 operate, the deployment of the fluidic and/or mechanical vortexgenerators 34, 36 and/or the positioning of the vortex generator (e.g.,yawing the mechanical vortex generator 34) to further facilitate thedelay of the onset of stall for the rotor blades 12.

For purposes of this discussion, each of the rotor blades 12 can bedivided into multiple sections so that load and aerodynamiccharacteristics of each section can be discussed and/or modeled and aninteraction of each and all of the sections can be assessed to providean efficient design for a complete (i.e., finite) rotor blade 12. Eachsection of the rotor blade 12 can experience differing load and/oraerodynamic characteristics for a myriad of reasons such as the airflow26 being unsteady, the rotor blade 12 experiencing increased airspeed atthe tip 20 (FIG. 1) of the rotor blade 12, twist and/or aeroelastics ofthe rotor blade 12, etc.

It will be appreciated in light of the disclosure that vortex generators30 in some sections can delay the onset of stall but in other sections,the vortex generators 30 can delay the onset to a lesser extent or notat all. In this regard, the separation of the boundary layer 32 is notalways an event that quickly occurs across the entire rotor blade 12.The boundary layer 32 can partially separate in some sections of therotor blade 12, while remaining generally attached in others. As aresult, the global effect can be a delay in the overall onset of stall,even though the airflow 26 over some sections of the rotor blade 12 canbest be characterized as in the stall condition.

A diagram 200 in FIG. 6 shows the effect of vortices generated by thevortex generators 30 on a value of lift coefficient versus a value ofangle of attack for the rotor blade 12 (FIG. 1). The value of angle ofattack can change in a periodic fashion from a nominal angle of attack202. A value of a maximum angle of attack 204 and a value of a minimumangle of attack 206 are shown as the angle of attack of the rotor bladevaries in the periodic fashion. A first data series 208 indicates avalue of the lift coefficient relative to values of angle of attack fora rotor blade without any vortex generators 30 implemented thereon. Asecond data series 210 indicates a value of the lift coefficientrelative to values of angle of attack for a rotor blade with one or morevortex generators 30 implemented thereon in accordance with the presentteachings. It can be shown that as the values of angle of attackfluctuate in the periodic fashion typically experienced by rotatingrotor blades 12, the effects of vortices from the vortex generators 30can provide relatively higher values of lift coefficient.

A diagram 300 in FIG. 7 shows the effect of vortices generated by thevortex generators 30 on a value of pitching moment coefficient versus avalue of angle of attack for the rotor blade 12 (FIG. 1). The value ofangle of attack can vary in a periodic fashion between a maximum angleof attack 302 and a minimum angle of attack 304. A first data series 306indicates a value of the pitching moment coefficient relative to valuesof angle of attack for a rotor blade without any vortex generators 30implemented thereon. A second data series 308 indicates a value of thepitching moment coefficient relative to values of angle of attack for arotor blade with one or more vortex generators 30 implemented thereon inaccordance with the present teachings. As the values of angle of attackfluctuate in the periodic fashion typically experienced by rotatingrotor blades 12, the effects of vortices from the vortex generators 30can provide relatively lower values of pitching moment coefficient.

The vortex generators 30, in accordance with the present teachings, canbe implemented on a rotor blade 12 that was otherwise initiallyconstructed without the vortex generators 30, such as in a retrofitprocess. At low Mach numbers (i.e., near or below M=0.4) and at highangles of attack, particularly in the unsteady flow environment of arotor blade, the vortex generators 30 can cause the boundary layer 32 toremain attached over a trailing edge region 50 (FIG. 3A) whileincreasing suction over a leading edge region 52 (FIG. 3A). This can beshown to result in higher lift, lower drag and lower local pitchingmoments compared to an airfoil without the vortex generators 30.Depending on the Mach number, however, the pitch rate of the rotor bladeand the increased airflow velocity over the leading edge region 52 canbe shown to result in a high and possibly detrimental velocity gradientover a section of the rotor blade 12. In some instances, a pocket ofsupersonic flow can be shown to occur ahead of the vortex generators 30.The high velocity gradient and/or the pocket of supersonic flow cancause separation of flow closer to the leading edge rather than thetrailing edge, which can negate the benefit of the vortex generators 30.

The vortex generators 30 can also be implemented on a rotor blade 1 2that is initially constructed with the vortex generators 30 so thatother characteristics of the rotor blade 12 can be modified and/ortailored to further benefit from the implementation of the vortexgenerators 30. In one example, the leading edge of the rotor blades canbe altered (e.g., adjust camber, bluntness, etc.) to slow the airflowalong the section of the rotor blade 12. Various shapes of the rotorblade 12 can be implemented with the vortex generators 30. Theconfiguration of the rotor blade 12 and the placement of the vortexgenerators 30 are based on a myriad of parameters that affect or definethe rotorcraft 14. In certain instances, a more desirable velocitydistribution over the rotor blade 12 in combination with a certainplacement of the vortex generators 30 can be achieved by adjusting thethickness, the camber, the leading edge radius of the rotor blade 12 andone or more combinations thereof.

Certain implementations can be determined by initiating an iterativedesign process to provide an optimized configuration of the vortex blade12, airfoil and the vortex generators. The improved velocitydistribution over the rotor blade in combination with certain placementof the vortex generators 30 can add to the reluctance of the boundarylayer 32 to separate from the rotor blade 12.

In one example and with reference to FIG. 8, an airfoil 400 can have abaseline bluntness 402 and an altered bluntness 404. The bluntness of aleading edge 406 of the airfoil 400 can be adjusted by adjusting theradius of curvature of the leading edge 406. The rotor blade 12 with thealtered bluntness 404 on the leading edge 406 can also include one ormore vortex generators 30 and therefore define an example of an airfoilthat has been modified to accommodate and benefit from the vortexgenerators 30 relative to an airfoil having the baseline bluntness 402to which a vortex generator 30 is simply attached.

In various examples of the present teachings, the vortex generators 30can establish a series of vortices. There can be a given number ofvortices and, moreover, the spacing, the direction, the phase, thestrength and one or more combinations thereof can be controlled totailor the vortex generators 30 to a suitable aerodynamic environment ormultiple environments typically encountered by the rotorcraft 14. At theleast the above parameters can be simulated and/or tested empirically onvarious airborne mobile platforms to produce one or more suitableconfigurations of vortex generators 30 to benefit the airborne mobileplatform.

The various aspects of the vortex generators 30 can be implemented tolower at least the oscillatory loads on the rotor controls, hub, andstructure of a rotorcraft 14. This can help to reduce componentwear-and-tear and increase the life of the rotorcraft 14. The vortexgenerators 30 can also be used to generally maintain the oscillatoryloads on the rotor controls, hub and structure of a rotorcraft 14 butcan be used to expand the performance envelope of the rotorcraft 14. Indoing so, the use of the vortex generators 30 can enable higher thrustlevels without exceeding rotor control load limits. Moreover, the vortexgenerators 30 can expand current flight envelopes of rotorcraft 14 andthereby achieve increased speed, altitude, vertical lift, maneuvercapability and combinations thereof.

In one aspect of the present teachings, the vortex generators 30 can beapplied to rotor blades 12 that can be included in a tail rotor of asuitable rotorcraft. By implementing the vortex generators 30 on thetail rotor, the maximum thrust produced by the tail rotor can beincreased, thereby increasing a low-speed yaw maneuvering capability ofthe rotorcraft 14. Moreover, rotorcraft 14 that have implemented thevortex generators 30 on the rotor blades 12 of a main rotor can also usethe vortex generators 30 on a tail rotor to, among other things, offsetthe yawing moment associated with the performance increase of the mainrotor.

In one aspect of the present teachings, the vortex generators 30 can beimplemented on each of the rotor blades 12 on the rotorcraft 14 (FIG.1). As shown in FIG. 9, the vortex generators 30 can define mechanicalvortex generators 34 that are arranged along the leading edge region 52of the rotor blade 12. Specifically, the vortex generators 30 can bevanes 500 that can be arranged in pairs 502 in the leading edge region52 so that each of the pairs 502 of the vortex generators 30 aredisposed at a location that is about 10% of a chord line 504 of one ofthe rotor blades 12 thereby defining a location 505 that is near theleading edge. Each of vanes 500 in a single pair 502 can be oriented onthe rotor blade 12 so that a leading edge 506 of each of the vanes 500in the pair 502 are pointed toward one another and thus can form anangle 508 that is, in one example, about fifteen degrees from the chordline 504 of the rotor blade 12. Put another way, a direction parallel toa vortex generator chord line 510 can establish the angle 508 with adirection that is parallel to the chord line 504 of the rotor blade 12.

In one example, each of the vanes 500 of the pair 502 can be spaced fromone another a distance 512 of about 0.25 inches (about 6.35 millimeters)measured from about the quarter chord of each vane 500. Each of pairs502 can be spaced from other pairs of vanes 500 on the rotor blade 12 adistance 514 that is about one inch (about 25.4 millimeters). Each ofthe vanes 500 can be about 0.2 inches (about 5.08 millimeters) long(i.e., along the vortex generator chord line 510) and can be about 0.1inch (about 2.54 millimeters) tall (i.e., a dimension normal from asurface 516 of the rotor blade 12. The thickness of the vane 500 can beabout 0.025 inches (about 0.635 millimeters).

In a further example, the vanes 500 can be configured for certainapplications, one of which can include a rotorcraft 14 having two mainrotors like a Boeing Chinook CH-47. In such an application, each of thevanes 500 of the pair 502 can be spaced from one another a distance ofabout 0.75 inches (about 19.1 millimeters) measured from about thequarter chord of each vane 500. Each of pairs 502 can be spaced fromother pairs of vanes 500 on the rotor blade 12 a distance that is aboutthree inches (about 76.2 millimeters). Each of the vanes 500 can beabout 0.6 inches (about 15.2 millimeters) long (i.e., along the vanechord line) and can be about 0.3 inch (about 7.62 millimeters) tall(i.e., a dimension normal from the surface 516 of the rotor blade. Thethickness of the vane 500 can be about 0.075 inches (about 1.91millimeters). It will be appreciated in light of the disclosure thatother configurations of the vortex generators 30 can be implementedbased on the airborne mobile platform and the mission for that airbornemobile platform.

While specific aspects have been described in this specification andillustrated in the drawings, it will be understood by those skilled inthe art that various changes can be made and equivalents can besubstituted for elements thereof without departing from the scope of thepresent teachings, as defined in the claims. Furthermore, the mixing andmatching of features, elements and/or functions between various aspectsof the present teachings may be expressly contemplated herein so thatone skilled in the art will appreciate from the present teachings thatfeatures, elements and/or functions of one aspect of the presentteachings may be incorporated into another aspect, as appropriate,unless described otherwise above. Moreover, many modifications may bemade to adapt a particular situation, configuration or material to thepresent teachings without departing from the essential scope thereof.Therefore, it may be intended that the present teachings not be limitedto the particular aspects illustrated by the drawings and described inthe specification as the best mode presently contemplated for carryingout the present teachings but that the scope of the present teachingswill include many aspects and examples following within the foregoingdescription and the appended claims.

1. An airborne mobile platform comprising: a plurality of rotating rotorblades operating in an airflow that forms a boundary layer on each ofthe rotor blades, at least one of said rotor blades including: a sectionthat encounters the airflow that includes an unsteady subsonic airflowhaving at least a varying angle of attack; and one or more vortexgenerators on said at least one of the rotor blades that generate avortex that interacts with the boundary layer to at least delay an onsetof separation of the boundary layer, to increase a value of an unsteadymaximum lift coefficient and to reduce a value of an unsteady pitchingmoment coefficient for said section.
 2. The airborne mobile platform ofclaim 1 wherein a shape of at least said section is altered to change avelocity distribution over said at least one of said rotor blades basedon an effect of placement of said one or more vortex generators andwherein said shape of said section is altered to change at least one ofa thickness, a bluntness, a leading edge radius, a camber and one ormore combinations thereof.
 3. The airborne mobile platform of claim 1wherein said one of the vortex generators includes a vane that extendsfrom a surface of said section.
 4. The airborne mobile platform of claim3 wherein said vane is adjustable relative to the airflow in a yawdirection.
 5. The airborne mobile platform of claim 3 wherein said vaneis operable to retract below said surface of said section.
 6. Theairborne mobile platform of claim 1 wherein said one or more vortexgenerators includes a jet in said section that at least one of extractsand injects a jet flow into said boundary layer.
 7. The airborne mobileplatform of claim 1 wherein said one or more vortex generators arepositioned at a location on one of the rotor blades that corresponds toabout ten percent chord of said at least one of said rotor blades. 8.The airborne mobile platform of claim 1 wherein said one or more vortexgenerators includes at least a first mechanical vortex generator thatdefines a vortex generator chord line and wherein a direction parallelto said vortex generator chord line establishes a first angle with adirection parallel to a chord line of at least one of said rotor bladeson which said first mechanical vortex generator is connected, said anglebeing about fifteen degrees.
 9. The airborne mobile platform of claim 8wherein said one or more vortex generators includes a second mechanicalvortex generator that defines a vortex generator chord line and whereina direction parallel to said vortex generator chord line of said secondmechanical vortex generator establishes a second angle with a directionparallel to a chord line of at least one of said rotor blades on whichsaid second mechanical vortex generator is connected, said second anglebeing about fifteen degrees and wherein a leading edge of said firstmechanical vortex generator and a leading edge of said second mechanicalvortex generator are inclined toward one another.
 10. The airbornemobile platform of claim 1 wherein said one or more vortex generatorsincludes a first mechanical vortex generator and a second mechanicalvortex generator that each define a vortex generator chord line that isinclined relative to a chord line of at least one of said rotor bladeson which said first and second mechanical vortex generators areconnected.
 11. The airborne mobile platform of claim 10 wherein aleading edge of said first mechanical vortex generator and a leadingedge of said second mechanical vortex generator are inclined toward oneanother.
 12. The airborne mobile platform of claim 1 wherein said one ormore vortex generators include a first pair and a second pair ofmechanical vortex generators, each of said mechanical vortex generatorsin said first pair are spaced from one another about 0.75 inches andsaid first pair and said second pair of mechanical vortex generatorsspaced from one another about one inch.
 13. The airborne mobile platformof claim 1 further comprising a controller that adjusts said one or morevortex generators based on at least a rotational position of at leastone of said rotor blades.
 14. The airborne mobile platform of claim 13wherein said adjustment of said one or more vortex generators includesmoving a vane between an extended condition and a retracted conditionand wherein said vane in said retracted condition is disposed below asurface of said section of said at least one of said rotor blades. 15.The airborne mobile platform of claim 13 wherein said adjustment of saidone or more vortex generators includes at least one of activating one ormore vortex generators, deactivating one or more vortex generators,changing a magnitude of a jet flow from one or more vortex generators,changing a frequency of a jet flow from one or more vortex generators,changing a pulse width of a jet flow from one or more vortex generatorsand one or more combinations thereof.
 16. The airborne mobile platformof claim 1 wherein said one or more vortex generators includes an arrayof closely spaced fluidic vortex generators.
 17. The airborne mobileplatform of claim 16 further comprising a controller operable to adjustsaid array of closely spaced fluidic vortex generators, wherein saidarray of closely spaced fluidic vortex generators establishes a firstset of fluidic vortex generators and a second set of fluidic vortexgenerators and wherein said adjustment of said array of closely spacedfluidic vortex generators includes at least one of activating said firstset, deactivating said first set, changing a magnitude of a jet flowfrom said first set, changing a frequency of a jet flow from said firstset, changing a pulse width of a jet flow from said first set and one ormore combinations thereof.
 18. A rotor blade for use on a rotating rotorexperiencing an airflow, wherein the airflow forms a boundary layer onthe rotor blade, the rotor blade comprising: a section that encountersthe airflow that includes an unsteady subsonic airflow having at least avarying angle of attack; and one or more vortex generators located neara leading edge, said one or more vortex generators that that delays anonset of separation of the boundary layer, wherein said vortex interactswith the boundary layer to increase a value of an unsteady maximum liftcoefficient and reduce a value of an unsteady pitching momentcoefficient for said section.
 19. The rotor blade of claim 18 wherein ashape of at least said section is altered to change a velocitydistribution over said rotor blade based on placement of said one ormore vortex generators and wherein said shape of at least said sectionis altered to change at least one of a thickness, a bluntness, a leadingedge radius, a camber and one or more combinations thereof.
 20. Therotor blade of claim 18 wherein said vortex generator includes a vanethat extends from a surface of said section.
 21. The rotor blade ofclaim 18 wherein said vortex generator includes a jet that at least oneof extracts and injects a jet flow through a surface of each of therotor blades and into the boundary layer.
 22. The rotor blade of claim18 wherein said vortex generator on each of the rotors blades ispositioned at a location that corresponds to about ten percent chord ofthe rotor blade.
 23. The rotor blade of claim 18 wherein said vortexgenerator includes a mechanical vortex generator that defines a vortexgenerator chord line and wherein a direction parallel to said vortexgenerator chord line establishes an angle with a direction parallel to achord line of the rotor blade on which the mechanical vortex generatoris connected, said angle being about fifteen degrees.
 24. A method forimproving performance of an airborne mobile platform having rotatingrotor blades, the method comprising: rotating rotor blades through anunsteady subsonic airflow having at least a varying angle of attack,each of said rotor blades having a first value of a maximum liftcoefficient in said airflow; generating vortices over each of said rotorblades; and establishing a second value of said maximum lift coefficientthat is greater than said first value of said maximum lift coefficientin said airflow due to said vortices.
 25. The method of claim 24 whereinsaid generating vortices over each of said rotor blades includes movingvanes from a retracted condition to an extended condition.
 26. Themethod of claim 24 wherein said generating vortices over each of saidrotor blades includes injecting and extracting a jet flow.
 27. Themethod of claim 24 wherein said vortices are generated near a leadingedge of each of the rotor blades.
 28. The method of claim 24 whereinsaid generating vortices over each of said rotor blades includesaltering a shape of said rotor blades to change a velocity distributionover said rotor blades based on placement of one or more vortexgenerators on said rotor blades.
 29. The method of claim 28 wherein saidshape of said rotor blades is altered to change at least one of athickness, a bluntness, a leading edge radius, a camber and one or morecombinations thereof.
 30. The method of claim 24 wherein said generatingvortices over each of said rotor blades includes placing a firstmechanical vortex generator and a second mechanical vortex generatornear a leading edge of each of said rotor blades and wherein a leadingedge of said first mechanical vortex generator and a leading edge ofsaid second mechanical vortex generator are inclined toward one another.